I have tried to get this setup up and running a few weeks ago, but at the time I
had issues with iSCSI failover (running on CentOS 5.4 VMs). Would you share your
pacemaker config ?

I have now also moved to NFS. From a perf point of view, a single VM on an NFS
datastore looks slower than on an iSCSI LUN, but I think NFS scales much better.
Even VMware recommends to go no further than 16 VM on a LUN, compared to 250 on
an NFS store.

The issue with NFS is that you can't go dual-primaries (unless you are NetApp,
probably). I haven't found a way to have 2 NFS servers running in parallel, and
exporting the same filesystem (due to NFS storing information locally in
/var/lib/nfs). As far as I can tell, even a cluster FS (OCFS2, GFS) wouldn't
help.

However, the problem can be solved by running two single-primary clusters. This
requires 4 VMs, and a slight admin overhead, but seems to work well.